Cleans up users adderssfield country

#### What's this PR do?
- Provides addressfield option to override address country with affiliate country
- Adds the option to chain of users address filters
- Changes registration form: replaces hardcoded country with affiliate country
- UK and Canada registration: removes hardcoded country in favor of common solution
- UK and Canada user creation on login: replaces hardcoded country with affiliate country

#### What are the relevant tickets?
- Closes #3328 
  ![screen shot 2014-11-11 at 11 34 03 pm](https://cloud.githubusercontent.com/assets/672669/5001244/b8bba1b6-69fc-11e4-8297-9d1843629106.png)
  ![screen shot 2014-11-11 at 11 33 33 pm](https://cloud.githubusercontent.com/assets/672669/5001243/b26ea38a-69fc-11e4-83b1-07fbd408e55b.png)
- Closes #3341
  ![screen shot 2014-11-11 at 11 35 07 pm](https://cloud.githubusercontent.com/assets/672669/5001248/be71a51a-69fc-11e4-89ba-239646794b28.png)
  ![screen shot 2014-11-11 at 11 50 46 pm](https://cloud.githubusercontent.com/assets/672669/5001355/9bc495b2-69fd-11e4-846b-1094faeb4f27.png)


 Nop. Now this is feature's normal state, on all sites. Affiliates are going to get its code from `dosomething_settings_get_affiliate_country_code()`, but US is still the default.